The overall theme of this research program is to describe cochlear nonlinearities in humans with normal hearing and with hearing loss. Cochlear nonlinearities are evident in many measures, all of which change as a consequence of damage to the peripheral auditory system. It is not possible to directly assess cochlear status in humans, but indirect acoustical and electrophysiological measures can be used to assess nonlinear behavior in responses that describe cochlear function. It also is possible to test the hypothesis that different measures reflect the influences from common underlying mechanisms. Studies included in this proposal will (1) increase our knowledge of underlying processes responsible for the relationship between sensitivity loss and otoacoustic emission generation, (2) lead to a better understanding of the consequences of damage to nonlinear mechanisms with respect to response latencies, (3) describe the changes in suprathreshold function following cochlear damage, and (4) lead to better diagnostic tests that identify the sensory cell population underlying hearing loss in humans. It is hypothesized that all of the phenomena to be studied as part of this research program may reflect different manifestations of a single underlying mechanism associated with cochlear nonlinearities. This unified interpretation will be evaluated with multiple correlational analyses and by manipulation of a simple cochlear model that has anatomical correlates. The melding of both basic and clinical studies will lead to advancements in knowledge of auditory function in humans with normal hearing and with hearing loss, leading to improvements in how hearing loss is identified, quantified and remediated. These improvements, being based on objective measures of threshold and suprathreshold function, will be especially helpful in the diagnosis and treatment of hearing loss in infants, young children, and patients with developmental delay.